1. Field of Invention
This invention relates to pollution control systems for internal combustion engines and more particularly to pollution control systems which utilize exhaust gas recirculation.
2. Prior Art
Since the amount of NO.sub.x emitted during acceleration of an internal combustion is great and the amount emitted during cruising speeds is small, the reduction effect of exhaust gas recirculation (hereinafter referred to as EGR) on the NO.sub.x contained in the engine exhaust gases is greatest during acceleration and only slight during cruising.
Since EGR generally has an adverse effect upon automobile performance and increases fuel costs, it is desirable that EGR be allowed to operate only during acceleration when the NO.sub.x reduction effect is greatest and that deterioration of performance be avoided during cruising speeds when the effect of EGR is slight.
In the prior art there exists several different systems which utilize exhaust gas recirculation, but these systems typically are exceedingly complex and utilize a myriad of vacuum operated valves, electromechanical valves, electronic sensors, etc. Besides their complexity, the prior art systems also operate with varying degrees of success and none has been found to be completely satisfactory and almost all cause some deterioration in engine performance.